maybeNotDoVCFallback(self,
res)
Override this in a subclass if you want to detect unrecoverable
checkout errors where clobbering the repo wouldn't help, and stop the
current VC chain before it clobbers the repo for future builds.

doVCFull(self)

parseGotRevision(self)

Override this in a subclass. It should return a string that represents
which revision was actually checked out, or a Deferred that will fire
with such a string. If, in a future build, you were to pass this
'got_revision' string in as the 'revision' component of a SourceStamp,
you should wind up with the same source code as this checkout just
obtained.

It is probably most useful to scan self.command.stdout for a string of
some sort. Be sure to set keepStdout=True on the VC command that you run,
so that you'll have something available to look at.